Functionally Graded Composites

Some applications require composites in which phases of radically different properties have to be joined, such as ceramics with metals or rigid hydroxyapatite with highly deformable collagen. Such applications include the constmction of space vehicles (with a ceramic layer outside and a metaUic stmcture inside a vehicle), biocomposites, gas turbine blades, machining tools, attack vehicle armour and many others. 

However, in the case of such components, there is a risk that excessively high residual thermal stress occurs, due to the very different thermal expansion coefficients, thermal conductivity. Young s modulus, etc., of the component phases. 

To avoid this, the composites consist of layers, with thicknesses in micrometers or millimetres, having a composition which is modified in stages (Fig. 3.13). This results in nearly continuous changes in properties throughout the composite. These composites are known by the acronym FGM, standing for functionally graded materials. 

---

Carrillo-Heian, E.M., Carpenter, R.D., Paulino, G., Gibeling, J.C., Munir, Z. (2001), Dense layered molybdenum disilicide-silicon carbide functionally graded composites formed by field-activated synthesis , J. Am. Ceram. Soc., 84, 962-968. 

Processing and Properties of Electrodeposited Functionally Graded Composite Coatings of Ni-Al-Al203... 

Olevsky EA, Wang X, Maximenko A, Meyers MA (2007) Eabrication of net-shape functionally graded composites by electrophoretic deposition and sintering modeling and experimentation. J Am Ceram Soc 90 3047-3056... 

Impedance spectroscopy has been a useful tool for separating the grain-interior and grain-boundaiy contributions in many electroceramics. [48,49] In most analyses, the brick layer model, which assumes electrically and dimensionally homogeneous grain and grain boimdary, was used. [7,13,24-26] (Fig.3(b)) However, the distribution of the and Pgh values can become spatially uneven for the specimens under an electric field [50-53] or with a functionally graded composition. [54]... 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

Suresh, S., and A. Mortensen, Fundamentals of Functionally Graded Materials Processing and Thermomechanical Behavior of Graded Metals and Metal-Ceramic Composites, Ashgate Publishing Co., Brookfield, VT (1999). 

Low, I.M., Skala, R.D. Zhou, D. (1996a) Synthesis of functionally-graded aluminium titanate/alumina composites. J. Mater Sci. Lett. 15, 345-347. 

Pratapa, S. Low, I.M. (1998) Infiltration-processed functionally-graded AT/alumina-zirconia composites II, Mechanical properties. J. Mater Sci. 33, 3047-3053. 

Skala, R.D. (2000) Development of a functionally-graded alumina/aluminium-titanate composite. PhD Thesis, Curtin University of Technology, Perth, Australia. 

In the development of functionally graded materials, there are two approaches. One is to eliminate the boundary of laminated-type composites, thereby eliminating discontinuities in the properties at the boundary. The other option is to make non-uniform distributions of dispersoids in a homogeneous composite, thus creating multiple functions within the material.2 Therefore, continuous variation in composition, microstructure and so on, results in change in properties as a function of position in the component.6... 

In compositional design of functionally graded SiAION ceramics, selection of cations and a P phase ratios are very important parameters. Type of cations affects the densification and diffusion between the layers. They also control the thermomechanical properties of SiAION ceramics. 

---